Basically they were all the same circuit up until the Rat II was introduced. The only difference in the First "Tone " control model was that the Tone control was wired backward from the Filter control versions. ALl of the early ones both Big Box and small box all have the same circuits in them. I have read some of the very first ones used Tantalum caps which may have affected the sound as the caps aged.

Also the New Rat II and Vintage Rat reissues are the exact same circuit just a different box. Both are True Bypass. I was actually down to visit Proco last weekend and got to seem them being built.

I also got to try a Proto of a new Rat they have coming out in January. It will be called the Solo. The picture is of the Proto when they were considering calling it the RatZo. The other picture is of Charlie Wicks who started Proco and is the CEO.

The SOLO model is supposed to have 4 knobs with both Bass and Treble contols that are somewhat interactive so you can get a somewhat scooped tone as well. The clipping diodes are going to be different and use asymetrical clipping.

The Tone knob models sound different ... or at least the one I was borrowing from a friend for a spell.
Less gain, less scoop.
I compared it to my early 80's big-box Filter model, which sounded absolutely identical to another big-box Filter knob that I found and sold.
I can't say which one I liked better as they're both great, depends on the app.
They are similar mind you, but the difference in my case was still clear.